[19] Phil. Mag., 1872.
Art. 78. The Electro-Magnetic Theory of Light.--We have seen ([Art. 71]) that light is due to a periodic wave motion of the Aether, and we have previously seen that heat is also due to a periodic wave motion of the Aether. Thus in the phenomena of light and heat, Aether is the medium in which the energy of light is stored, and by which it is transmitted in its passage from a luminous body, as the sun, until it comes into contact with a planet or satellite from which it is reflected, thus giving rise to light and heat. When, however, we come to deal with electro-magnetic phenomena, which are the results and effects produced by electricity and magnetism, we find certain phenomena similar to those that we find in relation to light and heat. Thus, when light is emitted by a luminous body, a certain amount of energy is given out by that body, and if such light is absorbed by another body, the latter becomes heated, a clear proof that it has received energy or motion from some outside source. From the time it left the luminous body till it reached the lighted or absorbing body, it must have existed as energy, that is, motion in the Aether. As we have already seen, Newton thought that the transference of energy was accomplished by the actual transference of certain small corpuscles or atoms given out by the luminous body, which conveyed the energy of the one body to the other. According to the wave theory of light, however, we find that the transference of energy is accomplished by a wave motion in the Aether, which is periodic both in time and space, by which wave motion the energy is transferred from the luminous to the illuminated body. Now every one is familiar with the effects of magnetism and electricity in some form or other, and such familiarity teaches that various kinds of work may be done by electricity. If an electric current be generated and allowed to flow through any circuit, as the ramifications, for example, of an electric-tram system, it can readily be seen that by the action of the current large masses or bodies as trams may be moved. To generate the current requires the expenditure of energy, and for the tram to be moved requires the transmission of that energy from the generating station till it reaches the body to be moved. By what means is such energy transmitted? because if it disappears at one place and reappears at another, it must have passed through a medium during the interval. It has been demonstrated that the medium which conveys the current from place to place is the Aether, so that as light is transmitted through space by the Aether, in a similar manner electric currents are transmitted through space also by the same medium. The discoverer of this great truth was Clerk Maxwell, and it was from the consideration of electro-magnetic phenomena that he was able to lay the foundation of that theory known as the Electro-Magnetic Theory of Light. In paragraph 781 of his greatest work[20] he says: “In several parts of this treatise an attempt has been made to explain electro-magnetic phenomena by means of mechanical action from one body to another by means of a medium occupying space between them. The undulatory theory of light also assumes the existence of a medium. We have now to show that the properties of the electro-magnetic medium are identical with those of the luminiferous medium.” He then points out that, “to fill all space with a new medium, whenever any new phenomenon is to be explained, is by no means philosophical”; and further adds, that “If it should be found that the velocity of propagation of electro-magnetic disturbances is the same as the velocity of light, and this not only in air, but in other transparent media, we shall have strong reasons for believing that light is an electro-magnetic phenomena.” In the wave theory of light we have seen ([Art. 70]), that two properties are necessary to any medium before it is capable of transmitting wave motion of any kind. Those two properties are elasticity and inertia. Water possesses these properties, and so can transmit ocean waves; air also possesses these properties, and so can transmit sound waves; and Aether, being matter, also possesses these properties ([Arts. 47] and [48]), and is therefore capable of transmitting light waves. The elasticity is essential in order for the medium to store up energy, and also to enable it to resume its original shape after deformation, while the inertia is necessary in order that the medium may transmit the impulse, and oscillate to and fro until the impulse received has been passed on. This elasticity and inertia may be well illustrated by the bending of a lath or cane. If we pull one end down, holding the other end quite still, we shall see that the lath oscillates to and fro until gradually it comes to rest. The elasticity of the lath allows it to be pulled out of its original position, and also enables it to rebound, while its inertia causes it to swing back again past its original position. Both combined together cause it to swing backwards and forwards till its energy is used up. If such a series of springs could be set in motion at equal intervals of space and time, we should then have a good illustration of a wave motion.
What analogy, may be asked, is there in electro-magnetic phenomena to correspond to this elasticity and inertia of the Aether, so essential to the propagation of light? Let us look at the familiar illustration of charging a Leyden jar. In charging a Leyden jar with electricity a certain amount of energy is spent, work is done, and the result is found in the electrified state of the jar. That which has actually been accomplished is the storing up of energy in the Aether around the jar. This storing up of energy is analogous to pulling aside the lath, and is the making use of the elasticity of the Aether, in order to produce a tendency to recoil. When the jar is discharged, which is analogous to letting go the lath, the Aether seeks to recover its former condition by discharging the energy it received. In these operations the elasticity of the Aether is called into play. After the jar is discharged, however, the recoil of the Aether produces a current, and the inertia of the current causes it to overshoot its original position, and for an instant the charge of the jar is reversed. The current now flows backwards in the same way that the lath returned back, and charges the jar as at first. This discharging and recharging continue backward and forward, so to speak, until all the energy which was originally given to the jar has been expended, and it resumes its normal condition. In this experiment the elasticity and inertia of the Aether have both been called into play, so that we see in this electrical experiment a similar illustration of the elasticity and inertia of the Aether, as manifested in the undulatory or wave theory of light. The question now arises, what are the corresponding properties as given by Maxwell in his electro-magnetic theory? In Art. 782 he writes: “In the theory of electricity and magnetism adopted in this treatise two forms of energy are recognized--the electro-static and the electro-kinetic--and these are supposed to have their seat, not merely in the electrified or magnetized bodies, but in every part of the surrounding space where electric or magnetic force is observed to act. Hence our theory agrees with the undulatory theory in assuming the existence of a medium which is capable of becoming a receptacle of two forms of energy.” Faraday, in his Experimental Researches, paragraph 3075, in referring to the character of magnetic phenomena external to the magnet, writes: “I am more inclined to the notion that in the transmission of force there is such an action external to the magnet, than that the effects are merely attraction and repulsion at a distance. Such a function may be a function of the Aether if it should have other uses than simply the conveyance of radiations” (light and heat). From this extract we learn that Faraday was also of the opinion that the Aether around a magnet or any electrified body was directly concerned in the propagation of the electric and magnetic forces, these forces according to Maxwell being of two kinds. From the illustration of the charging and discharging of the Leyden jar, we learn that aetherial electrical waves can be produced by electric means, and from the alternate charging and recharging of the jar we learn that these aetherial waves travel to and from the jar with a periodic wave motion. Here, therefore, we have an aetherial wave motion which is produced wholly by electricity, and yet which answers our definition of a wave motion of light, in that it is periodic both in time and space, and in that aetherial wave motion Maxwell states that two forms of energy are called into play, which he calls Electro-Static and Electro-Kinetic. These correspond respectively to the elasticity and inertia in the older theory of the wave motion of light. It was upon this basis that Maxwell built up his electro-magnetic theory. Even this theory does not tell us what is the exact character or nature of the periodic wave motion of the Aether. All it tells us is, that the electro-magnetic wave motion of the Aether is the same in nature and character as the wave motion which produces light and heat. Thus it shows that light and electricity have a common origin, and proves that light is nothing more or less than an electro-magnetic phenomenon. Maxwell gave a number of proofs in support of his theory. He showed that the velocity of the electro-magnetic waves was almost identical with the velocity of light waves, his results being as follows--
| velocity of electro-magnetic waves. | velocity of light in metres per sec. | ||||
|---|---|---|---|---|---|
| Weber | 310,740,000 | metres | per sec. | Fizeau | 314,000,000 |
| Maxwell | 288,000,000 | " | " | Foucault | 298,360,000 |
| Thompson | 282,000,000 | " | " | ||
From these figures it can readily be seen that the velocity of an aetherial wave, generated by electric means, is approximately the same as an aetherial wave generated by a luminous body. Thus one of the most important results of Maxwell's theory was to show that electro-magnetic disturbances produced in the Aether by an electrified or magnetic body might be propagated through space with a velocity equal to that of light.
It was left, however, for Professor Hertz to place the electro-magnetic theory of light upon a sure and certain foundation. The results are to be found in his work on Electric Waves, translated by Professor Jones, 1893.
In his paper on “The Velocity of Propagation of Electro-dynamic Action,” he gave definite and experimental proof of the hitherto theoretical fact that the velocity of the electric waves in air was exactly the same as that of light, whereas he found that in wires the ratio was not the same, being 4 to 7. This was afterwards found out to be an error by some experiments made at Geneva, when it was discovered that the propagation in wires was the same as in the air. Among his experiments Hertz succeeded in producing very short electric waves of 30 centimetres in length, that is, about one and one-fifth of an inch. According to Maxwell's theory, such a wave ought to behave exactly as a beam of ordinary light does. Hertz proved that this was the case, and published his proofs in his paper on “Electric Radiation.” In that paper he showed how such electric radiation was propagated in straight lines, like light, and that it could also be refracted and reflected. Thus he gave to Maxwell's electro-magnetic theory experimental demonstration, and placed it on a solid and immovable foundation. In summing up the results of this theory, we learn, therefore, that Hertz has conclusively proved that electric and magnetic effects are propagated through the Aether which fills all space with the same velocity that light is propagated. Further, he has conclusively proved the identity between light and electricity, and shown that electric and light radiations are essentially one and the same, and that they are both propagated by periodic wave motions of the Aether. Further, he has proved that the velocity of the propagation of light is the same as that of the electro-magnetic waves, and that these waves obey all the laws that govern light and heat. We have here, therefore, experimental proof of the identity between electricity and light, and in [Art. 69] we have also proved the identity of light and heat, so that we have now experimental proof that light, heat and electricity are all due to the periodic wave motions and vibrations of the universal Aether, which not only fills all space, but which surrounds every atom and every particle of matter throughout the whole universe. Having established, therefore, the identity of heat, light and electricity, and having proved that they are all due to the periodic wave motions or vibrations of the universal Aether, it must follow as a matter of necessity that wherever in interplanetary or interstellar space we find light or heat waves we must also find electricity. We have already seen that aetherial light waves flood all space, both interplanetary and interstellar space, so that in view of the identity of the aetherial light waves and aetherial electric waves, it follows that the aetherial electric waves flood all space in the same way, and at the same time. Wherever, therefore, we find the light waves, there we find the electric waves also; and it will be impossible to find the one without the other. Thus, throughout all space, and indeed throughout the universe, light waves will not be found apart from electric waves. They are as incapable of being dissociated as are light and heat waves. Now we have already seen ([Art. 64]), so far as the solar system is concerned, that the sun is the generator of all light and heat, and that these light waves speed from the sun on every side with a velocity of 186,000 miles per second. From the identity of light and electric waves, therefore, given to us by the electro-magnetic theory of light, it must follow that the sun is equally the source and generator of the electric waves. Not only so, but as the light waves flood all solar space, these electric waves, being identical with the light waves, must flood the solar system also. Thus we learn from Maxwell's theory as developed by Hertz, that not only is the sun the generator of light and heat waves which are poured forth into space continually with a velocity almost inconceivable, but at the same time the sun is pouring forth into space electric waves which travel outwards in spherical shells in the same way as light waves do, and with a similar intensity, as we shall see in the next chapter. Now let me ask the reader to ponder over the fact given to us by this electro-magnetic theory in its relation to the solar system, and endeavour to find out what such an application teaches us. Let it be remembered that we are looking for a Centrifugal force or motion, that is, a motion from a centre, which is to be the exact counterpart of the Centripetal force, i. e. motion to a centre; and further, that the Centrifugal motion must be a repulsive motion acting in the opposite way to the attractive power of the Centripetal force, that is, the attractive power of gravity. We have seen ([Art. 77]) that light possesses a repulsive power. We have now only to prove that electricity or the aetherial electric waves have a repulsive motion, which will be the easiest of all to prove, and then we shall have proved beyond the possibility of contradiction, the existence of that repulsive force referred to by Herschel in [Art. 24], which is to form the complementary and counterpart of the attractive power of gravity. If it can be proved that electricity does possess such a power, that is, a repulsive power, ever acting from a centre, then in view of the identity of light, heat and electricity, the correctness of the views we have advanced as to the repulsive power of light and heat will be proved beyond the shadow of a doubt, otherwise Maxwell's electro-magnetic theory of light is a fable and a myth, and Hertz' experiments were never performed. Further, if all electro-magnetic phenomena are due to the same aetherial medium which gives rise by its wave motions to light, heat and electricity, then we shall have discovered a medium which throughout the universe can by its wave motions transmit and propagate both repulsions and attractions, that is, the aetherial medium which is to be the physical cause of Universal Gravitation. In order to further develop and establish this point we will now consider the subject of Electricity as a Mode of Motion.
[20] Mag. and Elec.